The invention relates to an antilocking braking control system for a road vehicle having a hydraulic multiple circuit brake system with at least one closed brake circuit which is connected to an output pressure space of a brake booster in which a static pressure, proportional to the force of actuation, is generated. This pressure is connected to a primary chamber of a pressure modulator provided to control pressure reduction, pressure build-up, as well as pressure maintenance phases of the antilocking control system for the at least one wheel brake of the static brake circuit via an inlet valve, which is mechanically drivable into its open position. The wheel brake can be connected to, or alternatively closed off from, the primary chamber of the pressure modulator via an electrically controllable brake pressure control valve, formed as a 2/2-way solenoid valve.
The inlet valve, which is mechanically drivable into an open position, is located in a primary chamber of a pressure modulator for controlling pressure reduction and pressure build-up, as well as pressure maintenance phases of the antilocking control system, to at least one wheel brake of the static brake circuit. The wheel brake is connectable to, and alternatively closed off from, the primary chamber of the pressure modulator.
An electrically controllable brake pressure control valve in the form of a 2/2-way solenoid valve is connected between the primary chamber and the at least one wheel brake. The pressure modulator has a drive pressure space separated from the primary chamber by a modulator piston. The drive pressure space is connected in normal non-antilocking braking control mode to a pressure output source. The pressure output source provides and maintains a high pressure level of brake fluid.
An auxiliary pressure source provides a high output pressure brake fluid source connected to the driven pressure space, via an electrically controllable ABS function control valve, to supply high pressure brake fluid when the ABS valve is in its basic position. The high pressure brake fluid in the driven pressure space applies a force to the modulator piston which is biased against a pressure force by the action of a powerful restoring spring and the pressure prevailing in the primary chamber. The high pressure force displaces the piston to an end position, associated with a minimum volume of the primary chamber.
An electronic ABS control shifts the ABS valve from its basic position to an excited position, wherein the ABS valve controls a flow of pressure from the driven pressure space to a pressureless reservoir of the auxiliary pressure source, upon an activation of the antilocking control system in the sense of a brake pressure reduction at the at least one wheel brake of the static brake circuit.
The ABS valve is switched back to its basic position by the ABS Control during at least one of: a pressure build-up phase, a pressure maintenance phase of the antilocking control system for the at least one of the wheel brakes, or an operating phase of the antilocking control system in which all wheel brakes of the static brake circuit are closed off from the primary chamber of the pressure modulator due to brake fluid being fed back from the at least one wheel brake into the brake booster.
The primary chamber has a maximum volume corresponding to between 25% and 50% of a volume of brake fluid quantity which can be expelled back into the brake circuit by an actuation of the brake booster upon a maximum possible force of actuation. The ABS electronic control provides control signals necessary for appropriately controlling the ABS control valve in response to output signals which are characteristic of the motion behavior of the vehicle wheels as sensed by wheel speed sensors associated with the vehicle wheels.
A bypass control valve is opened when the modulator piston reaches at least one of its end positions, associated with a maximum volume of the primary chamber, or reaches the immediate vicinity thereof, to connect the brake booster to the at least one wheel brake through a bypass flow path.
An antilocking system of this type is the subject of Applicants' German Unpublished Patent Application P No. 36 37 781.3-21, which corresponds to U.S. patent application Ser. No. 07/116,867, filed Nov. 5, 1987, and now pending in Patent Examining Group 314.
In the antilocking control system described therein, the pressure modulator associated with the closed brake circuit, for example the front axle brake circuit, is of the type of a hydraulically driven pump having a feed pressure space (the primary chamber) which is movably defined and separated from a secondary chamber (the drive pressure space) by a modulator piston . The drive pressure space is connectable by means of an ABS function control valve to the high pressure output of an auxiliary pressure source, whereby the modulator piston is pushed into an end position, associated with a minimum volume of the primary chamber against the action of a powerful restoring spring. In this end position of the piston, an inlet valve, designed as a ball-seat valve, is held in its open position by a tappet supported on one side by a valve ball, and on the other side by the piston, as a result of which the output pressure space of the brake booster which is connected with inlet valve, communicatives with the primary chamber.
The two front wheel brakes are each connected to an output of the primary chamber via a brake pressure control valve designed as a 2/2-way solenoid valve. In the normal operating state of the brake system, i.e., when the brake system is not subjected to antilocking control, the high output pressure of the auxiliary pressure source is connected to the drive pressure space of the pressure modulator, and the piston of the latter is thereby held in the end position associated with a minimum volume of the primary chamber.
The piston and the auxiliary pressure source are so dimensioned that the piston remains in the above-mentioned end position, even if a braking action is performed with the maximum brake pressure. This maximum pressure is connected to the wheel brakes via the open inlet valve, the primary chamber and brake control valves when in their basic positions. If a locking tendency occurs at one of the braked wheels, which requires a pressure reduction in the wheel brake, then the brake pressure control valve of the other wheel brake is driven to its locking position, and the ABS control valve is driven to an excited position, in which the control pressure previously connected to the drive pressure space of the pressure modulator can be released towards a pressureless reservoir of the auxiliary pressure source. As a result, the piston is subjected to a displacement in the sense of a volume enlargement of the primary chamber, under the influence of the restoring spring and the brake pressure in the primary chamber. The inlet valve reaches its locking position after a small initial displacement of the piston in this displacement direction, and a pressure reduction at the brake to be controlled is achieved as the volume of the primary chamber is further enlarged.
The pressure modulator is so dimensioned that the maximum pressure reduction stroke of the modulator piston is sufficient for this, even if previously almost all of the maximum possible brake pressure was coupled into the wheel brakes and has to be reduced to a large degree by the control system. A pressure reduction of this kind, within one piston stroke, however, is no longer possible when the control system is effective on both wheel brakes.
For example, when a detected output signal of a position indicator, provided for the recording the piston position and characteristic of the piston position, no longer changes while the control system still continues to call for a pressure reduction at the wheel brakes, the brake pressure control valves of both wheel brakes are driven to their locking position.
The drive pressure space of the pressure modulator is again connected to the high output pressure of the auxiliary pressure source, as a result of which the modulator piston is then subjected to a displacement in the sense of a reduction of the volume of its primary chamber. The inlet valve has a construction corresponding to a non-return valve which opens as a result of the pressure building up in the primary chamber and brake fluid being expelled back out of the primary chamber into the tandem master cylinder. The pressure modulator functions in this phase like the return feed pump of known antilocking control systems.
After completing such a return feed stroke of the pressure modulator, the pressure reduction mode can be continued again by renewed displacement of the piston in the sense of an enlargement of the volume of the primary chamber. In order that the brake system, while operating without the antilocking control functions, remains functional even when the auxiliary pressure source has failed and the piston of the pressure modulator withdraws to its end position associated with a maximum volume of the primary chamber, a bypass valve designed as a ball-seat valve is provided which releases a second pressure medium flow path leading from the brake booster to the primary chamber of the pressure modulator as soon as the piston has reached a minimum distance, corresponding to the greatest part of its maximum stroke.
The valve ball, which up until then was pushed into its locking position by a spring-loaded closing element, is taken along with the piston from the point where the latter reaches a minimum stroke, so that the ball can separate from the valve seat.
Irrespective of numerous advantageous functional features of this antilocking control system in comparison to antilocking control systems of similar construction, an unfavorable feature is evident in the case where the auxiliary pressure source, although it has not failed completely, supplies only a reduced output pressure. There, the modulator piston can no longer be pushed into its end position, associated with a minimum volume of the primary chamber, by subjecting its drive pressure space to high pressure. This end position is also utilized for the non-controlled braking mode, so that the piston withdraws slightly under the influence of the brake pressure connected to the primary chamber, as a result of which the inlet control valve can reach its locking position even during the normal braking mode, and this leads to at least a restriction of the utilizable brake pressure, which of course is unacceptable.
Even more unfavorable is of course the case when the output pressure of the auxiliary pressure source is no longer sufficient to displace the piston, against the bias restoring force of the powerful restoring spring, at least close enough to its end position, associated with a minimum volume of the primary chamber. Such a displacement is necessary for the valve ball of the inlet valve to be lifted off its valve seat during at least part of its normal opening stroke, and for the bypass valve to remain held in its locking position by the still partially possible displacement of the modulator piston. If this displacement does not occur, it is no longer possible to brake at all with the brake circuit connected to the pressure modulator. The relatively complicated mechanical construction of the pressure modulator, including the inlet and the bypass valves, can also be seen as a disadvantage, albeit a less grave one.
The object of the invention is therefore to improve an antilocking control system of the type aforementioned that is less susceptible to malfunctioning and is of a simple construction, and in particular, one that ensures that the brake system remains functional even if the auxiliary pressure source fails. This insurance of function is obtained by utilization of a bypass flow path as will be explained subsequently.
This object is achieved according to the invention by having a bypass control valve means as a solenoid valve which is driven from a basic "O" position in which a bypass flow path is opened by an output signal from the electronic ABS control unit into an excited "I" position in which the bypass flow path is blocked, and wherein at least one pressure control valve means is movable to a basic "O" open position to allow flow between the primary chamber and the at least one wheel brake when, in the course of the pressure reduction, pressure build-up or pressure maintenance control phase of the antilocking control system, the high output pressure of the auxiliary pressure source falls below a minimum setpoint required for the displacement of the modulator piston into its end positions associated with a minimum volume of the primary chamber; and wherein this pressure control valve means is held in this basic "O" position. The design of the bypass valve as a 2/2-way solenoid valve, which can be driven by the electronic control, to the respective function position appropriate to the situation, irrespective of a particular displacement position of the modulator piston, makes it possible for braking to take place in every possible position of the modulator piston. To detect the malfunction situation, where the output pressure of the auxiliary pressure source has fallen below a minimum set point, a pressure signal switch characteristic thereof, can be utilized. Such a signal is usually provided for monitoring the pressure in a pressure reservoir of the auxiliary pressure source, which can be charged by the charge pump of the auxiliary pressure source.
Although the design of the bypass valve as an additional solenoid valve entails certain additional technical expenditure, this is, however, offset by a significant simplification of the construction of the pressure modulator, since a mechanically actuable bypass valve and the actuation closing elements necessary for its control are dispensed with. As a result, the pressure modulator can be manufactured with a considerably simpler construction, and hence more cheaply.
In the antilocking control system of the present invention, a position indicator is provided which generates electrical output signals characteristic of the displacement of the modulator piston. The output signals are connected to the electronic ABS control as further information inputs. The electronic ABS control generates signals characteristic of the displacement behavior of the modulator piston, as well as of the approach of the piston to its possible end positions, for switching the bypass valve into the excited locking position. It is possible to control the bypass valve appropriately to the situation using simple logical combinations. The additional technical electronic expenditure necessary with respect to this extension of the electronic control unit of the ABS can therefore be regarded as negligible.
This is particularly true when the inlet valve is designed as a one-way valve which is actuated in an opening direction by higher pressure in the primary chamber of the pressure modulator than in a valve chamber connected between the brake booster and the bypass path. A valving surface of the inlet valve is raised from a seat by an actuation tappet which is displaced by the piston of the pressure modulator, when the piston of the pressure modulator at least occupies the end position associated with a minimum volume of the primary chamber, or is displaced from the latter end position by less than a small fraction of 1/20 to 1/5 of total piston displacement.
The tappet passes through an inlet channel of the pressure modulator, which contains the valve seat, and the valving surface is held tightly against the valve seat by the restoring force of the valve spring and the pressure prevailing in a valve chamber in the bypass path. Here, the inlet seat valve can be attached directly to the modulator housing by use of a simple axial extension.
The connection of the wheel brakes of the brake circuit, subjectable to the control system via a one-way valve, to an additional bypass flow path leading to a pressure output of the brake booster, has the advantage that a rapid brake pressure reduction can take place in the wheel brakes when the drive quickly and drastically reduces the pedal activation force, while at the same time the ABS control valve is still functioning in a control phase in which the bypass valve and at least one of the brake pressure control valves is locked. For example, such can occur in a pressure build-up phase at one of the wheel brakes in which the pressure build-up is effected by expelling brake fluid previously received by the pressure modulator into the wheel brake subject to control. Thus, in a simple manner and after the shortest possible time lag, the brake pressure corresponds to the anticipated value supplied by the driver by actuating the brake pedal.
As a result of this situation, the ABS control valve has a flow path which provides connection of the pressure output of the auxiliary pressure source to the drive pressure space of the pressure modulator, when the ABS control valve is in the basic position through a throttle for restricting the pressure medium flow rate to the pressure modulator to a given value. Because of this, a changed rate of the brake pressure, in pressure build-up and pressure reduction phases of the antilocking control system, is obtained in an optimum relation to each other.
A further function control valve is provided between the auxiliary pressure source and the ABS control valve and is controllable by means of output signals of the electronic ABS control unit. The auxiliary pressure source includes a Pressure reservoir which is chargable by an accumulator-type charge pump and is maintained at a specific required output pressure level by a pressure-limiting valve connected between an output pressure of the pump and a return line leading to a pressureless reservoir. A second one-way valve is connected between the further function control valve and the ABS control valve.
This permits pressure maintenance phases of the antilocking control system and charging of a pressure reservoir within the framework of the auxiliary source which can be controlled in a simple manner. With the aid of this pressure limiting valve system, it is possible to operate the accumulator-type charge pump sparingly by bridging the pressure limiting valve, the opening pressure of which determines the reservoir pressure level.
A simple embodiment of the modulator piston and a position indicator suitable for monitoring its position, is achieved when the pressure modulator is designed as a step cylinder having bore steps of different diameters which merge into one another via a housing step. The piston has a correspondingly shaped two-step configuration with a larger piston step, defining with a large diameter cylinder step, the drive space and a smaller piston step defining with a smaller diameter cylinder step, the primary chamber of the pressure modulator. The piston has a central conical section which tapers in axial direction between its two steps. The position indicator has an actuating pin radially supported on the conical section. The position indicator is configured as a travel/voltage converter for converting radial displacements of the actuating pin into voltage signals which can be processed by the electronic control unit of the ABS. This arrangement permits an exact determination of the current position of the piston, as well as its speed of travel.
An additional feature of the invention is the restoring spring, which biases the modulator piston into its end position associated with a maximum volume of the primary chamber. It is designed as a helical pressure spring which is accommodated in a central blind bore of the modulator piston in a section of its length, approximately corresponding to an axial extension of the smaller piston step and the central conical section of the piston. As a result, the restoring spring still develops a desirable amount of force even in the end position of its piston, i.e., the end associated with a maximum volume of the primary chamber of the pressure modulator.
When the modulator piston is supported in the housing of the pressure modulator, an additional restoring spring, the restoring force of which is of a maximum at the position associated with a minimum volume of the primary chamber, also biases the piston as it is displaced increasingly away from this minimum volume position. The force of this additional restoring force reduces to a zero value at approximately 1/4 to 1/8 of the total stroke of the piston. A considerable restoring force is thus available in the particular one stroke region of the piston directly adjacent to the end position associated with a minimum volume of the primary chamber. The two restoring springs have a suitable dimensioning wherein the sum of the maximum values of the restoring forces of the biasing springs and the force resulting from a maximum output pressure of the brake booster acting upon the piston in the primary chamber, all operate in the sense of an enlargement of the primary chamber volume, and are of a value which is at least less than, or approximately equal to, the force in the driven pressure space which displaces the piston into its end position associated with a minimum volume of the primary chamber when the modulator piston is subjected to the output pressure of the auxiliary pressure source. Such a control system allows for counterphase control, i.e., the brake pressure can be built up in one wheel brake, while brake pressure is reduced at the other wheel brake.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.